Casting of metal



Patented Nov. 22, 1938 UNITED STATES PATENT OFFICE CASTING F METAL No Drawing. Application December 9, 1935, Se-

rial No. 53,610.

4 Claims.

This invention relates to the casting of freely oxidizable metal, particularly magnesium and alloys having a high magnesium content.

The objects of the invention are to provide a 5 method of casting freely oxidizable metal which prevents the metal to be cast from oxidizing in the mold, with the avoidance of any detrimental efi'ect of the molding material on the appearance or composition of the casting; also to provide a m composition of this kind which can be used without either preparatory drying of the molding material or baking of the finished molds, and which remains flt for frequently repeated use.

For the casting of freely oxidizable metals or 15 alloys. and more particularly of magnesium and its alloys, sand molds are as a rule employed. This practice is attended, however, with very considerable diiliculties, on account of the presence of water, seeing that more particularly magne- 20 slum and alloys high in magnesium when in the molten state react explosively with water or steam. To overcome this drawback the art concemed has developed along two concurrent lines.

Recourse was first had to an expedient that 5 had long been used in foundry work in general (cf. for example U. S. Patent specification No. 198,852 from 1878) for the purpose of avoiding the necessity for working with baked sand molds, which is costly and yet does not ensure continum ously good results,namely to the employment of non-aqueous liquids, and more particularly hydrocarbon oils, for plasticizing the sand. A proposal of this nature is to be found in the U. S. Patent specification No. 1,363,384 (to Bakken) 35 which describes the employment-of such binders in conjunction with molding sand freed from water, and more particularly also for the casting of magnesium; A method of the same nature forms the subject matter of the German Patent 376,739

40 the specification of which mentions different molding materialsas equivalents, viz. sand, coal powder, calcined magnesia, carborundum, anhydrous alumina, chamotte meal (grog), and the like.

45 In contrast to these attempts, which have had no influence on technical development, a method has been provided, by the addition of certain protective substances to molding sand, which has attained very considerable technical significance 50 for the casting of magnesium. This method of working took its rise from measures for the improvement of baked sand molds.

Since even with the employment of these expensive molds the castings not infrequently showed 55 burned spots the proposal was made of applying In Austria May 31, 1935 ,to the baked sand mold, in the form of a blackwash, substances having the common characteristic of giving off, on contact with the molten metal, gases or vapors which do not react with magnesium, either themselves or in the form of 5 their oxidation products (German Patent 368,- 906). This proposal did not make any essential contribution to the art either, but only had an indirect influence thereon, since it was subsequently discovered that substances of this nature are also capable of rendering practicable the far simpler but hitherto unfeasible method of casting magnesium in green sand molds. In the German patent specification 384,137 describing this method there are given as examples of such protective substances elemental sulphur, bicarbonates, oxalates, and boric acid, to which substances urea became added later. Following this discovery, various classes of protective substances came to be successfully used for this same purpose. As examples are mentioned: Ammonium salts in general, ammonium fluoride, ammonium bifiuoride or salts containing ammonium fluoride in a complex form in chemical combination, or mixtures of ammonium fluoride or of such complex salts with substances of an acid nature, hydrofluoboric acid or volatile salts of this acid, metal fluosilicates such as sodium or magnesium fluosilicate, and metal fluorides.

However, this method of casting magnesium in green sand molds, which has been very considerably improved by the additional protective substances found out by different inventors, is still attended by a serious drawback. The heat conductivity 0! molding sand is poor in itself, to the detriment of the finely crystalline structure and the mechanical properties of the castings dependent thereon. If there then be added to the sand or applied as blackwash to the surface of the finished mold, volatilizing substances which, on the casting of the metal, provide a cushion of gas between the mold and the casting, the cooling of the casting becomes so delayed that thicker parts of the cast exhibit coarsely crystalline structure, while at transitions from thicker to thinner 5 cross-sections micro-flaws (hair line cracks) occur. Both these detects have a very unfavorable influence on the mechanical properties of the tinished products. The known expedients at present available to the art for increasing the heat conductivity of the mold (the incorporation with the molding materials of finely divided additional materials of high conductivity, the employment of molds furnished with chills) not only increase the costs of production but also do not lead with certainty to the regular attainment of perfectly satisfactory results. Thus in particular. there is the danger, when casting in molds furnished with chills, of burned spots and blow-holes becoming formed on the casting in consequence of the condensation of water vapor on these chills.

In accordance with the invention, for the casting of light metals and their alloys in temporary molds, in conjunction with the known protective substances commonly employed to permit of working with green molding sand, there are employed as the main molding material dry metal oxides or oxidic ores or metallurgical products which are poor in silica such as for example magnesium oxide or burned magnesite, chromic oxide or chromite, these substances being converted into the plastic state in a known manner with the aid of a non-aqueous binding agent of the type of the hydrocarbon oils. Metal oxides of the nature specified are of very considerably higher heat conductivity than quartz sand: thus for example the thermal conductivityof magnesia is twice as high as that of sand. Moreover,

the specific heat per unit of volume of these oxides is also very considerably higher than that of quartz sand. The following table of figures, in which A denotes thermal conductivity, specific heat, 7 volumetric weight. and c. v the specifi'c heat per unit of volume, gives the coefiicients of heat transfer of quartz sand, sintered magnesite, and chromite in the form of grained powders that have been shaken up for a minute's duration.

Xcal

M R Kcal Kcal kg 0.

a h.m.c.kg.c.""dm* 'amsc.

300 c400? Quartz sand 0. 32 0. 210 1. 47 0. 309 Bintered magnesite 0. 68 0. 275 2. 54 0. 698 Chromite 0. 40 0.22 2.56 0.563

Thanks to the considerable superiority of the specified molding materials as regards heat transfer properties heat is so rapidly withdrawn from the castings that the delay in cooling inevitably attendant on the employment of protective substances cannot exert any detrimental eifect. Moreover, in consequence of the employment of dry molding materials in conjunction with non-aqueous plasticizing agents, that is to say with practically entire exclusion of water, it becomes possible to particularly good advantage to employ protective substances which, on. comingin contact with the molten metal, give ofi only a slight quantity of gases and vapors or which remain practically undecomposed. so that the substantially stronger quenching action of the molding material may take effect with hardly any weakening, in spite of the presence of the protective substances. The possibility of managing with such protective substances is also valuable in anotherrespect. The impoverishing of the molding material in protective substance, which inevitably occurs as a consequence of abundant evolution of gases and vapors, not only involves higher cost of production in consequence of increased consumption of protective material, but also introduces an undesirable element of uncertainty, since it happens that spoiled castings with burned spots and other defects are obtained owing to a drop in the amount of protective substance present below the admissible limit.

is obtained a surprising improvement in the nae-- ammonium fluoride. Ammonium fluoride, which gives off large quantities of gas at casting temperature, and thus by no means belongs to the preferred protective substances in the sense implied by the present invention, was selected for these comparative tests in order to demonstrate particularly clearly the superiority of the method according to the present invention. Tensile test specimens of 17, 20, 22, and 27 mm. diameter were cast and turned down to 16 and 18 mm. S represents the tensile strength in kgs/mm, and E the elongation in percentages.

Diameter of test piece castings 17mm. 20mm. 22mm. 27mm.

S E S E S E S E Quartz sand 16.4 6. 2 l5. 7 5.8 15.1 5. l 13. 7 4.5 Dead burned magnesite l9. 9 9. 18.3 7.8 17.3 7.0 15.8 5. 8 Chromite 19.2 8.6..-.

It will be seen that the specimens cast in dead biuned magnesite molds give the highest strength test and modulus of specific extension, followed fairly closely by thevtests of the specimens cast in chromite molds, while the specimens cast in green sand molds show very considerably lower figures for tensile strength and elongation. Be-

sides, there is obtained the very appreciable advantage from the practical foundry point of view that, with the method according to the invention, the specified results are obtained with very small risers; when casting in green sand molds, the results obtained in accordance with the invention cannot be reached even if the riser gates set up on the-top of the tensile test specimens be increased by 50% in length.

In addition to the advance achieved by the inter-communicating parts of the invention combining for a common result, the substitution of metal oxides or oxidic ores for sand also brings with it the advantage that the drawbacks resulting in the case of sand molds, from the high silica content of the sand, are obviated. Especially magnesium and magnesium alloys are not chemicall'y indifferent to the silica contained in the sand. Since under practical working conditions in foundries it is impossible to prevent molding sand with certainty from adhering to risers and gates, as a consequence thereof, when this scrap is melted down again, silica passes from this molding sand into the metal, and leads to the formation of silicides which have a deleterious effect on the metal. As is shown by the fact that molding sand with a high silica content has been preferred for the production of molds for the casting of magnesium (cf. Irresberger, Giesserei- Zeitung, XIX, 1922, p. 600; German Patent 567,823, page 1, line 56), insuflicient attention has been devoted hitherto to this disadvantage attended with sand molds. These drawbacks are also obviated by the use of the specifiedmolding materials which are free of or poor in silica.

Among the protective substances primarily contemplated for use in the present process on account .of the absence or reduction of gas formsmagnesite molds provides perfectly reliable protection of the metal from the atmosphere. Still better protective action is obtained by the addition of magnesium fluosilicate to the molding material, even 1% of this salt being sufficient to ensure the obtaining of perfectly faultless castings, without the necessity for any additional expedients. The fluoborates are also flt protective substances for use in the present method; they are however considerably more expensive than the metal fluosilicates,

The specific effect of such magnesium attacking protective substances by which a gas cover is not produced, is based upon an increase in the surface tension of the molten metal.- Thus for examplev magnesium,'when in contact with mold walls containing or yielding the specified protective substances, flows quite differently from when it is in contact with other mold walls. Whereas otherwise in casting there becomes formed a jet which runs like water with a rippled surface and consequently evinces a lively tendency to become black and to burn, the casting stream when in contact with molds of the nature provided by the present invention acquires a smooth surface which is best described by likening it to a mercury meniscus. In this manner the tendency of the magnesium to form compounds with oxygen and nitrogen, and to ignite, is cancelled. This modification of the manner of flowing, which entails very definite and obvious advantages, can

only be eifected with the use of protective substances which react with magnesium.

In order to facilitate the making up of the dry molding materials with the non-aqueous plasticizing agent into the moldable state, and at the same time to obtain a higher degree of plasticity, it is advisable to employ jointly other molding materials which are capable of serving to a certain extent as binding agents for the main molding material. There can be employed with equal success either additional substances of an inorganic nature such as clay and bauxite, or substances of an organic nature such as pulverized carbon or graphite, or both. In any case, however, the proportion of the main molding material poor in silica which shows a high heat conductivity must largely predominate, and should preferably not be below 90%.

The plasticizing of the molding material can be further regulated by compounding the main molding material, or the additional substances, or both, from coarser and finer particles. Making up the molding material in this manner also provides the possibility of varying the nature of the molds to suit the requirements of the casting. In an extreme case, the molding material is composed of coarsely disperse components and components of a colloidal degree of dispersiveness. In this manner the addition of foreign matter to the molding material of high conductivity can be dispensed with altogether, in which case, in-

(1) Dead burned magnesite of the composition, approximately of 85-90% MgO, 3-5% silica,

3-4 iron oxide, 1% aluminium oxide, and 1-3% calcium oxide is mixed, in a grain size of -0.5 mm., with 4% of clay and 1% of coal of approximately the same grain size, and plasticized with the aid of-asphaltic petroleum in a proportion of about 3% of the whole or of an equal amount of an artificial mixture of hydrocarbon oils with bitumen, there being added to the mixture as protective substances either 3% of sodium fluosilicate or 1% of magnesium fluosilicate. The mold is made from this mass in the usual manner, and its surface blackwashed for example with an alcoholic suspension of sodium fluosilicate or magnesium fluosilicate, or dusted with one of these protective substances. Casting is effected at the usual temperatures.

(2) In place of dead burned magnesite there is employed as the main molding material in the mixture given in Example (1) chromite of the composition, approximately, of 40-50% chromicoxide, 14% iron oxide, 14% aluminium oxide, 14% magnesium oxide, and 7% silica, in the same grain size as before. Otherwise the directions given in the first example remain unaltered.

It is known to prepare molding compositions, for the casting of ingot iron or mild steel, from a mixture of basic material very poor in bound and free silica, together with binding agents. Dead burned magnesite has been more particularly recommended as being suitable for this purpose. As molding material for the casting of iron and steel, magnesia is superior to molding materials containing silica, by reason of its higher refractoriness, since, in contrast to the silicates contained in sand, it does not melt at the casting temperatures of ingot iron or mild steel, and therefore forms no readily fusible slags. What is, therefore, intended in the casting of steel by the employment of magnesite as the main molding material, is to avoid the formation of readily" fusible silicate slags. In contrast thereto, the casting of light metals proceeds at temperatures far below the melting point of the silicates contained in sand. It follows that the requirements made of the nature of the a starchy material such as corn flour; this mix ture was then tempered with water to the desired consistency and packed into molds. The so made molds had finally to be baked to drive oiI excess moisture (cf. U. S. specification No. 1,525,519). This proposal made in connection with chromium alloys had no influence on the technical development of casting light metals,

especially magnesium, and their alloys, particu= larly in non-baked molds, and this was the less the case since, as already mentioned, sand with a high silica content has been preferred as molding material in the casting of readily oxidizable metals such as magnesium.

What we claim is:

l. The herein described improvement which comprises casting magnesium and high-grade magnesium alloys in an oil-tempered temporary mold made throughout chiefly from a molding material selected from the group consisting oi magnesium oxide and chromium oxide, while bringing a small proportion of sodium fiuosilicate into contact with the surface of the molten metal within the mold.

2. The herein described improvement which comprises casting magnesium and high-grade magnesium alloys in an oil-tempered temporary moldmade throughout chiefly irom a molding material selected from the group consisting of magnesium oxide and chromium oxide, while bringing a small proportion of magnesium fluosilicate into contact with the surface of the molten metal within the mold.

3. The herein described improvement which Patent roe-2,157,315,

KONRAD ERDMANN, ET AL,

anemia comprises casting magnesium and high-grade magnesium alloys in an oil-tempered temporary mold made throughout from a molding material selected from the group consisting of magnesium oxide and chromium oxide, this oxide forming substantially more than three-quarters of the whole, while bringing into contact with the surface of the molten metal within the mold a small proportion of a protective substance of the group consisting of salts of fluosilicic acid and hydrofiuoboric acid-:montaining a solid metal as the cation.

4. An oil-tempered temporary mold for casting magnesium and high-grade magnesium alloys containing as the largest component in every part a molding material selected from the group consisting of magnesium oxide and chromium oxide and having incorporated therewith a small proportion of a protective substance of the group consisting of salts of fluosilicic acid and hydrofluoboric acid containing a solid metal as the cation.

KONRAD ERDMANN.

FRI'IZ HANSGIRG. GEORG SCmCI-ITEL.

November 22 1958 o It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page )4, first column, line 6, after magnesimnfi" insert the following paragraph;

The term magnesiumoxide whenever used in the specification and in the appended claims means not only the oxide itself but also products ofburning native minerals containing mgnesium oxide as essential constituent The term chromium oxide? is used. to include native ore containing a considerable amount-of chromium oxide such as chrome iron'oreo and that the said Letters Patent should heread with this oorreoti on therein that the same may corfiorm to the record of the case in the Patent Officeo S-igied and sealed. this Birth day of January; A; Di, 1959i.

v(Seal) Henry Van arsdaie Acting Commissioner of Patentss I 

